The present invention provides a drive system for a variable diameter rotor blade system, and more particularly to a harmonic gear and spool system to selectively extend and retract the variable diameter rotor blade system.
A tilt rotor or tilt wing aircraft typically employs a pair of rotor systems which are pivotable such that the rotors may assume a vertical or horizontal orientation. In a horizontal orientation (i.e., horizontal rotor plane), the aircraft is capable of hovering flight, while in a vertical orientation (i.e., vertical rotor plane), the aircraft is propelled in the same manner as conventional propeller-driven fixed-wing aircraft.
Variable Diameter Rotor (VDR) systems are known to provide distinct advantages. That is, when the plane of the rotor is oriented horizontally, the rotor diameter is enlarged for improved hovering efficiency and, when oriented vertically, the rotor diameter is reduced for improved propulsive efficiency.
One known example of a VDR blade assembly provides an outer blade segment configured to telescope over a torque tube member. A retraction/extension mechanism includes a planetary gear arrangement which is selectively drivable in either direction by a control shaft coaxially mounted within the rotor system drive shaft. The control shaft is selectively driven to selectively rotate a reeling assembly and wind/unwind a cable attached to each outer blade segment. Controlling the extension and/or retraction of the outer blade segments relative to their torque tubes thereby varies the rotor diameter.
Each VDR blade assembly extends and retracts through rotation of the single reeling assembly. Disadvantageously, rotation of the single reeling assembly may result in relative differences in the position of each VDR blade assembly relative to the others. Such differences in the relative positioning may result in disbalance of the rotor system.
Accordingly, it is desirable to provide a VDR retraction/extension system which is light and compact while minimizing the potential for disbalance in the rotor system.
The VDR retraction/extension system according to the present invention includes a drive housing mounted at the root of each blade. The housing mounts the blade to a hub assembly and a spool assembly, a harmonic gear set and an electric motor. The spool assembly includes a pair of counter rotating spools each of which drive a respective cable. Each cable extends from the spool assembly through the interior of the inboard rotor blade section, along the longitudinal axis thereof, and around a pulley. The pulley is mounted to the outboard rotor blade section.
A cable end of each cable is attached to the inboard rotor blade section. The load applied by centrifugal force which operates to telescope the outboard rotor blade section relative the inboard rotor blade section is shared between each cable portion such that each cable portion carries only one-fourth the total blade centrifugal load. The relatively high centrifugal force load is thereby distributed through multiple components and redundant paths which decreases the fatigue and load requirements necessary for each member to advantageously provide for smaller diameter cables and a reduction in the overall size and weight of the drive system.
In operation, the electric motor drives the harmonic gear set which rotates the counter rotating spools. Rotation of the spools causes the cables to be wound onto or off their respective spool consequently effecting retraction/extension of the pulley and the attached outboard rotor blade section relative the inboard rotor blade section. As each blade drive system is independently driven by a separate electrical motor, each independent VDR blade assembly is independently positionable. That is, each VDR blade assembly on the hub may be individually positioned.
The present invention thereby provides a VDR retraction/extension system which is light and compact while minimizing the potential for disbalance in the rotor system.